Microwave (3 to 30 GHz) and millimeter-wave (30 to 300 GHz) circuits are widely used for high data rate communications; for sensing and ranging applications, and the like. It is common in microwave (and millimeter-wave, hereinafter called microwave) circuits to perform tuning to limit bandwidth and to optimize performance. A common type of tuning element is a mechanical stub tuner or a double-stub tuner, which consists of one or two branch or "stub" transmission lines in parallel with a through transmission line such as a coaxial transmission line. Shorti-circuits coupled to the stub transmission lines are adjusted closer to or farther from their junction to the through transmission line for impedance matching. Similarly, a portion of a transmission line similar to a resonant cavity can be tuned by a sliding short-circuit or plunger to provide a bandpass characteristic or to define the frequency of an oscillator.
It is often desirable to remotely control the operating frequency of a microwave system. This has been accomplished in the past by the use of mechanical short circuits remotely operated by a motor such as a solenoid actuator, selsyn (synchro) or the like. However, motor-driven short circuits in addition to the problems of intermittent wiper contacts associated with any mechanical sliding short circuit also have reliability problems relating to the motors and motor drives. For this reason, electronic tuning has become popular. Electronic tuning is accomplished by means of electrically variable elements.
A saturable reactor is an inductor with a magnetic core which affects the inductance. A bias current flowing through the core may drive the core into a region in which it is partially saturated, thereby changing the incremental inductance of the reactor. Saturable reactors tend to require a substantial number of ampere-turns for the bias current, and therefore tend to be relatively large and unsuited for high frequencies. Electronic tuning may also be accomplished by the sue of varicaps or varactor diodes coupled to the circuit to be tuned. A bias voltage is applied to the varicap diode to control its capacitance and thereby tune the resonant circuit. The use of the varicap diode, however, requires that the remainder of the circuit to be tuned have an inductive characteristic so that resonance can be achieved.
Antennas are tuned in two distinct fashions. The first and more basic manner is to change the physical dimensions of the structure, as by making dipole elements physically longer to tune from a higher frequency to a lower frequency. This is done, for example, in the common "rabbit-ear" television set-top antenna. This type of tuning maintains constant aperture in relation to the wavelength, and results in maintaining the same input impedance at the antenna feed terminals, and also results in maintaining the same radiation pattern, and therefore the same directive gain, at all frequencies. The second type of tuning associated with antennas involves varying the reactance as a function of frequency of a variable reactance element which is coupled to the antenna. For example, a dipole element has an input impedance which includes a series equivalent capacitive reactance, the magnitude of which increases with decreasing frequency. This might be "tuned" in the second manner with a variable series inductor, the reactance of which is selected to cancel the antenna capacitive reactance. This second type of tuning, however, allows the input impedance of the antenna to change with frequency (the remaining radiation resistance decreases with decreasing frequency), and the current distribution on the antenna also changes with frequency, resulting in changes in the radiation pattern and therefore in the directive gain.
It would be desirable to have the ability to move the physical position of a short circuit directly by electrical means without the intervention of an electromechanical converter such as a motor or a solenoid-actuated plunger.